When one does something they know they shouldn't do.
Often used in poker terms, where someone calls hands they shouldn't, but applicable to several life situations.
Origins come from zoo patrons who feed the monkeys, ignoring the signs prohibiting such conduct. Usually in return the monkeys throw their feces at them.

"Wow... Patrick just lost like half his chip stack.." 'Yeah.. he keeps feeding the monkey."

"I'm so full.. but it's ALL YOU CAN EAT." "You're going to pay for it later if you keep feeding the monkey man..."